The present invention relates to light emitting diodes. In particular, the invention relates to light emitting diodes that emit in relatively high frequencies within the visible spectrum (e.g., blue and violet) and that are used in conjunction with phosphors that convert some of the light generated by the LEDs into complementary colors that together with the LED light produce white output.
Light emitting diodes (LEDs) are a class of photonic semiconductor devices that convert an applied voltage into light by encouraging electron-hole recombination events in an appropriate semiconductor material. In turn, some or all of the energy released in the recombination event produces a photon.
Light emitting diodes share a number of the favorable characteristics of other semiconductor devices. These include generally robust physical characteristics, long lifetime, high reliability, and, depending upon the particular materials, low cost.
A number of terms are used herein that are common and well-understood in the industry. In such industry use, however, these terms are sometimes informally blended in their meaning. Accordingly, these terms will be used as precisely as possible herein, but in every case their meaning will be clear in context.
Accordingly, the term “diode” or “chip” typically refers to the structure that minimally includes two semiconductor portions of opposite conductivity types (p and n) along with some form of ohmic contacts to permit current to be applied across the resulting p-n junction.
The term “lamp” is used to designate a light emitting diode that is matched with an appropriate electrical contact and potentially a lens to form a discrete device that can be added to or included in electrical circuits or lighting fixtures or both.
As used herein, the term “package” typically refers to the placement of the semiconductor chip on an appropriate physical and electrical structure (sometimes as simple as a small piece of metal through which the electrical current is applied) along with a plastic lens (resin, epoxy, encapsulant) that provides some physical protection to the diode and can optically direct the light output. The package often includes a reflective structure, frequently formed of a polymer within which the diode rests. Adding a lens and electrical contacts typically forms a lamp.
Appropriate references about the structure and operation of light emitting diodes and diode lamps include Sze, PHYSICS OF SEMICONDUCTOR DEVICES, 2d Edition (1981) and Schubert, LIGHT-EMITTING DIODES, Cambridge University Press (2003).
The color emitted by an LED is largely defined by the material from which it is formed. Diodes formed of gallium arsenide (GaAs) and gallium phosphide (GaP) tend to emit photons in the lower energy (red and yellow) portions of the visible spectrum. Materials such as silicon carbide (SiC) and the Group III nitrides (e.g., AlGaN, InGaN, AlInGaN) have larger bandgaps and thus can generate photons with greater energy that appear in the green, blue and violet portions of the visible spectrum as well as in the ultraviolet portions of the electromagnetic spectrum.
In some applications, an LED is more useful when its output is moderated or converted to a different color. In particular, as the availability of blue-emitting LEDs has greatly increased, the use of yellow-emitting phosphors that down-convert the blue photons has likewise increased. Specifically, the combination of the blue light emitted by the diode and the yellow light emitted by the phosphor can create white light. In turn, the availability of white light from solid-state sources provides the capability to incorporate them in a number of applications, particularly including illumination and as lighting for color displays.
Several types of structures are currently used for color conversion in white-emitting LEDs. In one structure and related technique, the LED chip is placed on a package and then substantially or entirely covered with a polymer resin that carries a dispersed phosphor and that forms the lens portion of the LED lamp. Although this is a straightforward structure and process, it produces a relatively high variation in color across the diode.
In another technique, the phosphor is deposited directly upon or very near the chip surface, after which the lens resin is applied to fix the phosphor in place. This produces better color uniformity (lower CCT variation), a brighter output, and less undesired scattering. The corresponding disadvantage, however, is that the phosphor must be positioned precisely during the fabrication process, thus increasing the cost of the process and of the resulting diodes.
In a third option, the phosphor is applied in a “mini-glob;” i.e., as a small amount of resin carrying a dispersed phosphor that is applied only to the surface of the LED chip. In a separate step, polymer resin without phosphor is added to produce the final lens and package. The mini-glob technique is relatively easy from a fabrication standpoint, tends to be less expensive, and offers a good compromise as between the dispersed resin technique and the phosphor-on-chip technique. The corresponding problem, however, is that the geometry of the chip and the mini-glob (e.g., FIG. 1) results in a higher than desired emission of unconverted blue light. The mini-glob technique also depends on defining a meniscus of the resin that holds the phosphor. The outer edge of the chip is the easiest manner of defining such a meniscus, but this produces a lack of output uniformity based upon the absence of phosphor at the edge of the meniscus.
In a fourth option, a resin and phosphor are cast or molded into a pre-form (e.g., in the shape of a solid rectangle). The pre-form is then positioned adjacent the LED chip during the fabrication process. This produces a bright and dense output, but represents a relatively difficult fabrication. Accordingly, the fabrication and diode costs are relatively high. Additionally, the pre-form technique produces a diode geometry that tends to suffer from the same blue leakage as the mini-glob technique. Another problem with the pre-form technique arises from the current spreading fingers that are typically used on an active layer, particularly a p-type active layer. These fingers tend to prevent the pre-forms from resting flush on the diode chip. The resulting gap between the pre-form and the diode surface allows high angle blue light to escape without interacting with the pre-form.
Representative descriptions of several of these structures are set forth in commonly-assigned and copending application Ser. No. 60/745,478 for, “Side View Surface mount White LED,” the contents of which are incorporated entirely herein by reference. Other representative packages include (but are not limited to) U.S. Patent Application Publication No. 20050199884.
The phosphors themselves raise another potential problem. As generally well understood in the art, a phosphor is a solid material that absorbs photons of one frequency and then emits photons of a different (typically lower energy) frequency or range of frequencies. When blue light emitting diodes are used to produce white light, yttrium aluminum garnet (YAG), often cerium doped, represents a useful and exemplary phosphor. In use, YAG absorbs the blue frequencies such as those emitted by Group III nitride LEDs and converts the energy into a range of lower frequencies, with yellow being predominant. The combination of blue light from the LED and yellow from the phosphor produces an overall white emission.
Conventionally, phosphors are produced by mixing the relevant precursors and sintering them under pressure at relatively high temperatures (e.g. 1000° C.) and then mechanically milling the sintered product. This produces a powder that can be incorporated into the resin lens on an LED chip.
In the LED context, phosphor particles smaller than a certain size tend to avoid dispersing properly in the encapsulant, tend to exhibit a higher proportion of surface defects, and are less efficient in terms of white conversion and output. Thus, the phosphor particle size (based on the rough diameter across the particle) should be more than at least about 1 micron (μm) and preferably greater than 2 μm to maximize efficiency. In many LED applications, phosphor particle sizes in the 2-25 μm range are generally preferred.
These sizes are, however, large enough to be proportionally similar to certain dimensions of the LED chip. For example, and as illustrated in FIG. 1, larger particles are difficult or impossible to position at the edge of a mini-glob thus exacerbating the blue leakage problem.
The blue light problem is also exacerbated because in a diode formed from a plurality of epitaxial layers, the majority of light is emitted in a generally vertical direction; i.e., if the surface of the diode chip were considered to be horizontal, then most of the light is emitted within about 70° degrees of a line perpendicular to the horizontal surface. As a result, the phosphor particles are generally positioned to interact most efficiently with the more perpendicular emissions.
Additionally, the power emitted at angles closer to the horizontal (i.e., within about 20° of the horizontal surface) is generally lower than the power emitted vertically which adds to the difficulty in balancing the conversion of the blue light with the phosphor.